In order to assist startup or allow quick activation, diesel engines, various types of sensors, etc. employ a glow plug, a heater for a sensor, a heater for a fan, and the like. For example, in a diesel engine, air taken into a cylinder is compressed, and fuel is injected into the air whose temperature has increased as a result of adiabatic compression, whereby a resultant air fuel mixture spontaneously ignites and burns. However, in a case where such a diesel engine is started in winter or in a cold environment or a like case, since the temperatures of outside air, the engine, etc. are low, it is not easy to heat, only by means of compression, the air within the combustion chamber to a temperature required for spontaneous ignition. In order to overcome such a problem, a glow plug is used in such a diesel engine as means for igniting fuel.
A known heater which is used as a heater for a glow plug, a heater for a sensor, a heater for a fan, or the like has a structure in which a heating resistor element formed of, for example, an electrically conductive ceramic is embedded in an electrically insulative ceramic substrate. Specifically, Patent Document 1 discloses a ceramic-heater-type glow plug in which a resistor element formed of different types of electrically conductive ceramics which differ from each other in temperature coefficient of resistance is embedded in a substrate formed of an electrically insulative ceramic. As described above, Patent Document 1 proposes provision of a ceramic-heater-type glow plug which has quick heating performance and a self temperature controlling function, by means of combining resistor elements having different resistivities.
In the case of a glow plug, in order to realize quick heating performance and perform fine temperature control, a controller is used to control supply of electricity to the glow plug. However, at the time of startup, the voltage of a battery may drop in some cases, with a resultant failure to supply a sufficiently high voltage to the glow plug. In order to overcome such a drawback, a glow plug having low resistance may be used. However, in this case, since the resistance of the glow plug at room temperature is low, a large rush current flows when the supply of electricity is started. This problem can be solved through combined use of different materials having different resistances. Specifically, the resistor element may be configured such that only a front end side portion (heat-generating portion) of the resistor element is formed of a material having a relatively high resistivity, and a rear end side portion (including lead portions) of the resistor element is formed of a material having a relatively low resistivity. However, since this configuration increases cost, if possible, it is desirable to realize quick heating performance through sole use of a single material.
Patent Document 2 discloses a ceramic heater designed to reduce power consumption. The disclosed ceramic heater is characterized in that a heat-generating portion and lead portions of the ceramic heater are formed of the same electrically conductive ceramic, and the ratio of cross sectional area therebetween is determined to fall within a predetermined range. The document states that this configuration reduces power consumption. However, when the ratio of cross sectional area is increased, the surface temperature of a support member varies greatly among positions in its cross section. This problem can be mitigated by proper setting of the ratio of cross sectional area. However, when the temperature at the surface of the support member (substrate) is desired to be more uniform, the temperature of the interior (resistor element) of the support member must be increased excessively such that a portion on the surface of the support member which is low in temperature is heated to such a degree as to provide a satisfactory heating function of the ceramic heater. In such a case, energization durability (the durability of the ceramic heater as determined through a durability test in which the ceramic heater is energized repeatedly) may drop. That is, since a tradeoff relation exists between power consumption and energization durability, improving the power consumption and the energization durability simultaneously is actually difficult although its technical significance is large.
Incidentally, in the case of the ceramic heaters disclosed in Patent Documents 1 and 2, their heat-generating portions (a “first heating element 20” in Patent Document 1 and a “folded portion 3d” in Patent Document 2) assume a shape as shown in FIG. 9 such that a relatively long heat-generating front end portion 50 formed into a U-like shape is disposed along and in the vicinity of the outline of the substrate. Since it has been assumed that such a shape allows uniform, efficient heating of the substrate to thereby provide excellent quick heating performance and reduce power consumption, the heat-generating portion is formed into a U-like shape such that it is disposed along and in the vicinity of the outline of the substrate. However, when the present inventors made a resistor element having a shape different from the conventional shape assumed to provide excellent quick heating performance and reduce power consumption, the inventors found that, contrary to their expectations, the resistor element that they made has excellent quick heating performance, can reduce power consumption, and has improved durability.
Further, in recent years, a ceramic heater for glow plug has been demanded to have improved heating performance and durability and to further reduce power consumption. In particular, such a ceramic heater has been demanded to further reduce power consumption, while securing a sufficient amount of heat radiation in order to prevent deterioration in the startup performance of an engine. In addition, there has been increasing demand for a ceramic heater which has an excellent durability, can realize a temperature increasing performance such that the heater can reach 1000° C. within 1 sec upon supply of a small amount of power (also called “super quick temperature raising”) in order to contribute to new engine control, and can maintain such temperature increasing performance even when the power supply voltage drops to, for example, about 7 V.
Patent Document 1: Japanese Patent No. 3044632
Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 2006-24394